DC-to-DC converter has been widely used in various electronic apparatus to regulate the DC level of the supplied voltage, especially to level the voltage up or down at a desired value.
FIG. 1 shows a simplified conventional multi-phase DC-to-DC converter 10, which comprises multiple channels 12 connected in parallel to the converter output Vout. In each channel 12, a high-side MOSFET 122 is coupled between the input voltage Vin and a phase node 126, a low-side MOSFET 124 is coupled between the phase node 126 and ground GND, and the MOSFETs 122 and 124 are alternatively switched by drive signals UV and LV to generate a phase current I flowing through an inductor L to charge a capacitor Co to thereby generate an output voltage Vout. Due to the requirements to the operating voltage and cost for electronic apparatus, recent arts try to replace the low-side MOSFET 124 with normally-on Junction Field-Effect Transistor (JFET) or normally-off JFET to reduce the conductive impedance and cost. However, the high-side MOSFET 122 is only possible to be replaced with normally-off JFET, but not with normally-on JFET, it is for the reason that at the moment of the converter 10 powered on or powered off, the high-side drive signal UV cannot respond in time, such that if a normally-on JFET is used to serve as the high-side switch, the input voltage Vin will be short to the output Vout at the moment of powered on or powered off, and thereby to damage the load circuit coupled to the output Vout. A normally-on JFET is referred to the one that is conductive between its source and drain when the voltage difference between its gate and source is zero (i.e., Vgs=0), and will turn off when this voltage difference is less than a negative threshold voltage VtN (i.e., Vgs<VtN), while a normally-off JFET is referred to the one that is cut-off between its source and drain when Vgs=0, and will turn on when Vgs is greater than a positive threshold voltage VtN (i.e., Vgs>VtN). It is well known that a normally-on JFET has a lower conductive impedance than that of a normally-off JFET.
On the other hand, in the two-step multi-phase DC-to-DC converter proposed in U.S. Pat. No. 6,839,252 issued to the co-inventor, the supply voltage is first converted to a second voltage by a single-phase or multi-phase voltage converter, and then a multi-phase voltage converter is used to convert the second voltage to the output voltage, such that the ripple is minimized, and the product of the inductance and capacitance is reduced. As a result, the efficiency is improved. However, the prior two-step multi-phase DC-to-DC converter did not teach to use JFET to serve as the high-side switch of the voltage converter.